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Hurricane Irma Forecast Discussion Number 37
2017-09-08 10:46:56| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Fri Sep 08 2017 000 WTNT41 KNHC 080846 TCDAT1 Hurricane Irma Discussion Number 37 NWS National Hurricane Center Miami FL AL112017 500 AM EDT Fri Sep 08 2017 Microwave images and data from an Air Force Reserve Hurricane Hunter aircraft indicate that Irma is currently undergoing an eyewall replacement cycle. A recent GMI overpass showed an 50 nmi wide outer eyewall, with the inner eyewall weakening. The Hurricane Hunter aircraft reported peak 700-mb winds of 147 kt in the outer eyewall near 0500 UTC, and maximum SFMR winds were in the 125-130 kt range. Based on these data, the initial intensity is reduced to 135 kt. Irma is forecast to remain in a favorable warm water, light shear environment for the next 36-48 h. The intensity guidance shows a slow weakening during this time, but Irma is expected to remain at least a Category 4 hurricane until landfall in Florida. After landfall, a fairly quick decay in maximum winds is expected due to land interaction and increased shear, although Irma's large wind field is likely to still produce hurricane-force winds over a large area. There are two caveats to the intensity forecast. First, some additional weakening could occur during the eyewall replacement, followed by re-intensification as the cycle completes. Second, the ECMWF, UKMET, and NAVGEM forecast a track over or close to the coast of Cuba that is not currently a part of the track forecast. If this occurs, Irma could be weaker than currently forecast along the later parts of the track. The initial motion is west-northwestward or 285/14. Irma should maintain this general trajectory for the next 24-36 h as it moves along the southwestern side of the subtropical ridge. After that time, the guidance is in good agreement that the ridge should break and allow Irma to turn north-northwestward to northward. There remains some spread between the models on when the turn will occur, with the GFS/Canadian being on the eastern side of the guidance and the UKMET/NAVGEM on the left side. The ECMWF, Florida State Superensemble, and the HFIP Corrected consensus are in the middle of the guidance envelope, and the new track forecast is in best agreement with those models. Overall, the new forecast track is similar to the previous forecast, with minor westward adjustments at 36 and 48 h. KEY MESSAGES: 1. Irma is an extremely dangerous Category 4 hurricane and will continue to bring life-threatening wind, storm surge, and rainfall hazards to the Turks and Caicos Islands and the Bahamas through Saturday. Heavy rainfall is still possible across portions of Hispaniola through today. Hurricane conditions will also spread over portions of the north coast of Cuba, especially over the adjacent Cuban Keys through Saturday. 2. Severe hurricane conditions are expected over portions of the Florida peninsula and the Florida Keys beginning Saturday night. Irma is likely to make landfall in southern Florida as a dangerous major hurricane, and bring life-threatening storm surge and wind impacts to much of the state. A Hurricane Warning is in effect for southern Florida, the Florida Keys, Lake Okeechobee, and Florida Bay, while Hurricane Watches have been issued northward into central Florida. 3. A Storm Surge Warning is in effect for southern Florida and the Florida Keys. A Storm Surge Warning means there is a danger of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours. This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials. A Storm Surge Watch has been issued north of the Storm Surge Warning area for portions of the central Florida coast. 4. There is a chance of direct impacts in portions of Georgia, South Carolina, and North Carolina, but it is too early to specify the magnitude and location of these impacts. FORECAST POSITIONS AND MAX WINDS INIT 08/0900Z 21.7N 73.8W 135 KT 155 MPH 12H 08/1800Z 22.1N 75.7W 135 KT 155 MPH 24H 09/0600Z 22.6N 77.8W 135 KT 155 MPH 36H 09/1800Z 23.3N 79.4W 135 KT 155 MPH 48H 10/0600Z 24.5N 80.4W 130 KT 150 MPH 72H 11/0600Z 28.0N 81.5W 90 KT 105 MPH...INLAND 96H 12/0600Z 33.0N 84.0W 40 KT 45 MPH...INLAND 120H 13/0600Z 36.0N 87.0W 25 KT 30 MPH...INLAND $$ Forecaster Beven
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